Vacuum-venturi process of rapidly digesting pulp



Aug. 17, 1965 w. L. SCHMIDT VACUUM-VENTURI PROCESS OF RAPIDLY DIGESTING PULP 2 Sheets-Sheet 1 Filed July 24, 1961 PAPER BLEACHER BEATER BLACK LIQUOR SOLVENT DIGESTER WOOD CHIPS CHEMICAL RECOVERY FIG. 5

CIRCULAR STEAM SUPPLY DIGESTER SHELL INVENTOR. WALTER L. SCHMIDT Aug. 17, 1965 w. L. SCHMIDT VACUUM-VENTURI PROCESS OF RAPIDLY DIGESTING PULP 2 Sheets-Sheet 2 Filed July 24. 1961 o w m I l I I I l l I I S Hi l l IIIIHIIIHII H ENL P L D W I L T N P 3i 4 O 1 T T H C m U S P m a w I l I I I I I l I l I II A E I T G N R E A T M H N E%m $3.5m. E C E E3350 L .6 V P D w S R E R S N M 2 U NP O 6 CN I Om F TU I I I I v I I I I I I l I ll WU M W INVENTOR WALTER L. SCHMIDT TUBE DIAMETER- 0 TO V TUBE DIAMETER-D 0.4 D

United States Patent VACUUM-VENTURE PRQNIESS 0F RAPIDLY DEGESTING PULP Walter L. Schmidt, KingGeorgaVa assignor to Waverly Associates, inc, King George, Va., a corporation of Virginia Filed July 24, 1961, Ser. No. 126,1ti6 2 Claims. (Cl. 162-53) The present application relates to an improved process of digesting pulp and to a novel digester therefor.

Heretofore, the art has proposed a variety of processes for the production of pulp and has suggested various types of digesters; Generally speaking, the art in practice, has actually used processes and digesters for industrial operations such as those disclosed in various prior publications, such as The Manufacture of Pulp & Paper published by McGraw-Hill (1922, 1927) and Pulp and Paper Chemistry and Chemical Technology (Second Edition) written by James P. Casey and published by lnterscience Publishers, Inc. (-1960). As is well known, these prior processes and digesters had shortcomings including the extended period of time required to complete the operation of producing finished pulp, the yield of pulp, the quality of pulp, the loss of volatile constituents evaporated during the digestion, etc.

Although attempts have been made to overcome the foregoing difficulties and other disadvantages, none, as far as I am aware, was entirely successful when carried into practice commercially on an industrial scale.

It has been discovered that better quality of pulp and higher yields thereof can be made when wood chips are treated by the present invention involving the recirculation of the delignifying media through a venturi and the delignification of the wood chips within a relatively short period of time without degradation of the fibers under partial or full vacuum or controlled differential pressures. Such controlled differential pressures may be effected, for instance, by maintaining a maximum pressure in the steam jet of the venturi and a minimum pressure in the space above the fluid mass in the digester.

It is an object of the present invention to provide an improved process of digesting pulp so as to produce increased yields which are higher than those usually obtained from conventional processes employing standard digesters.

Another object of the invention is to provide an improved process of digesting pulp whereby a better grade of pulp is produced and fiber degradation is substantially eliminated.

The invention also contemplates providing an improved process of digesting pulp in which the volatile constituents are steam distilled by means of venturi action and are recovered by condensing the condensate in a suitable manner.

It is a further object of the invention to provide a process requiring less expensive digester shells due to the fact that there is no thermal shock on discharging the fluid mass nor is the digester shell required to withstand relatively high pressures, such as about 60 to about 150 lbs. per square inch.

The invention further contemplates providing operations producing delignifying media so saturated with dissolved lignin, hemocelluloses and other soluble components that such media may be burned in reclamation furnaces without first concentrating in evaporators which are expensive and which cost approximately /3 of the entire cost of pulping.

Still another object of the invention is to provide a novel digester and venturi combination provided with a jet for liquid, gaseous, or fluid mass to impart a pressure to such fluid mass and give the same momentum and also did i 3% having the jet nozzle or throat area of the venturi or both provided with spiral or helical vanes to cause rotation of the moving fluid mass and to impart a better action to the suspended cellulosic material.

It is likewise within the contemplation of the invention to provide a continuous process by suitably arranging several digesters in series.

Among the further objects of the present invention is the provision of a rotating fan installed in direct operative association with the vapor outlet of the novel digester in such a manner that the blades of the fan will rotate under the vapor outlet and above the top of the cooking liquor to depress, coalesce and/ or return the foam of cooking liquor having foaming tendencies to the fluid mass in the digester thereby obviating the use of chemical depressants which may interferewith subsequent recovery operations.

Other objects and advantages will become apparent from the following description taken in conjunction with the accompanying drawing, in which:

FIG. 1 is a flow sheet of the general operations involved in the novel process of producing pulp;

FIG. 2 depicts an elevational view of a novel digester, partly in section, used in carrying the invention into industrial practice;

FIG. 3 is a sectional view taken on the line 33 of FIG. 2;

FIG. 4 illustrates a modification of the novel digester shown in FIG. 2; and

FIG. 5 is a sectional FIG. 4.

Generally speaking, the present invention contemplates an improved process of digesting pulp involving a more rapid pulping of wood chips or cellulosic materials resulting in a maximum yield of cellulose under partial vacuum and moderate temperatures from the treatment of larger quantities of raw materials than heretofore with the same amount of chemicals used as delignifying agents or solvents. It is likewise within the contemplation of the invention to digest pulp under differential pressures. Within the digester, the fluid mass may be under a moderate pressure, say about 40 to about p.s.i. (pounds per square inch). The steam for the venturi jet is admitted at a high pressure, say about 250 to about 600 p.s.i.. The rapid digestion is obtained as if under vacuum by the rapid and complete circulation produced by the unique venturi action of the novel combination embodying the present invention described more fully hereinafter.

The improved process of digesting pulp includes the usual operation of charging the digester with wood chips or cellulosic materials and then sealing the digester in the ordinary manner. After charging, the digester is filled with an appropriate amount of Water and at least one delignifying agent or solvent of suitable character. To exhaust the air from the digester as well as any remaining in the wood chips or cellulosic material, etc, the operation of a vacuum pump is started. As a partial vacuum is created, steam is admitted to the digester via a modified venturi located in circulation pipes mounted within the digester. By so doing, the steam fulfills a plurality of functions, including heating, the fluid mass, conducting delignifying medium and cellulosic material and circulating the same from the bottom to the top of the digester and applying pressure on the fluid mass to effect deeper penetration of the solvent solution and then circulating the fluid mass from the top to the bottom of the digester to efiect a complete circulation of delignifying medium and to effect deeper penetration of the liquor and a scouring effect. The circulation of the fluid mass is continued until samples of the liquid and the cellulosic material indicates a complete delignification. In the new process, this period of time is relatively short and may be as low as about several view taken on the line 5-5 of minutes, say about 3 to about minutes, after starting the steam through the venturi with well-graded chips. The temperature of the fluid mass is maintained below about 165 F. by increasing the vacuum in the digester. The quality of the vapors is better and yield larger than those obtained by conventional processes and no preparatory heating time is required. Moreover, the volatile materials drawn off by the suction pump are recovered in a condenser as valuable by-products.

In carrying the invention into practice, it is preferred to utilize the procedures set forth hereinafter in detail. While the present process lends itself to the several standard pulping operations now in use in modern paper making, the choice of pulping media is determined by the desired pulp to be produced. The customary liquor of the plant that plans to utilize this invention may be used, no changes are necessary in the preparation or recovery systems heretofore used in pulping plants. Old digesters may be converted to embody the many advantages of the novel digester which will be described hereinafter and which is illustrated in the drawings.

In operating a sulphate or soda digester, for example, sulficient wood chips are charged into the digester and liquor added to make a fluid mass. Usually, about six to about eight times the weight of liquor to the dry weight of the chips are used. The digester is then sealed in the usual manner. The charge weights of liquor and chips are at atmospheric temperature and preferably not above 165 F. the preferred maximum temperature of the present process. The digester is connected to a suitable condenser and suction or vacuum pump so that a partial vacuum may be drawn and maintained at between about 12 and about 18 inches during the digesting or cooking. Steam is admitted to the venturi jets at a pressure of about 75 psi. or greater, if available, or in a super-heated condition. The cook or digestion proceeds immediately after a flow of the fluid mass is started. Thorough agitation is accomplished by having the lower or suction end near the lowest points of the digester. The wood chip component of the fluid mass tends to become fibrous first in the venturi tube throat or point of greatest constriction, then on rapidly expanding under a minus pressure or partial vacuum after passing the throat or point of greatest constriction, and further upon discharge upon the surface of the fluid mass in the digester. To secure further defibration, the digester is discharged to a blow tank via a venturi in the discharge duct which effects a result almost equivalent to the conventional discharging operation under pressure. Some further defibration may be required in the washing heaters and refiners.

The cook or digestion is continued until a sample of the liquor and the wood chips shows the desired delignification of the wood chips. As this process uses an apparent greater volume of liquor to dry Wood than conventional usage, it might be at first glance through to be undesirable. However, as the maximum concentration of lignin and other soluble solids has not been reached in the first or initialcharge, the liquor is repeatedly used until maximum concentration of lignin and soluble solids is obtained. Usually about four to about six passes are required and the net result is a more concentrated liquor to be recovered. For instance, about two to about five times the net weight of dry chips are processed with customary amounts of liquor to chip ratio. Dilution of the liquor due to excess water in the charged Wood chips is removed by a slight increase of the vacuum whereby optimum amount of dissolved lignin and sugars permit direct burning of the liquorto recover chemicals.

As live steam is passed through the venturi, dilution might result. However, this is overcome by maintaining the digester under vacuum via a condenser and pump at from about 12" to about 16" of mercury. The vacuum and the temperature are controlled and a constant fluid mass level and a maximum concentration of solubles are maintained in the digester. In addition, the volatile constituents of the wood undergoing cooking or digesting, distill off with the steam and are condensed and separately recovered by use of suitable equipment such as a pump, a suitable centrifuge, etc.

It will be understood by the art that slight modification may be employed to fit variable conditions existing in different pulp mills. However, this would not change the validity of the basic principle of applying a vacuum or partial vaccum within a closed vessel embodying the structural features of the novel digester illustrated and described herein and the use of the modified venturi tube to provide pressure on the fluid mass. In conventional existing digesters, it may be desirable to use a plurality of ventur-i tubes, such as 2, 4 or even 6 venturi tubes, which may be about 12 to about 24 inches in diameter. The use of additional venturi tubes will further shorten the time cycle and will prevent stagnant areas or pockets in the fluid mass. the use of suitable electronic equipment such as electric eyes, relays and appropriate auxiliaries to control the vacuum by the level of the fluid mass or other suitable means. Furthermore, several digesters may be connect.- ed in series to give a continuous process. Such a series system is contemplated in carrying the invention into actual commercial practice.

It has been found as a result of experiments and tests that a fluid mass which will flow through a venturi tube requires about the same as, or one and one-half times more than, the usual weight of digester liquor. It should preferably be at least six to about eight times the weight of dry cellulosio material charged or suflicient liquor to permit the fluid mass to circulate around the digester and to flow through the venturi tubes.

The temperature at about 16" vacuum in the present cycle is about 135 F. For a temperature of about 176 F., only 10 of vacuum is needed. The amount of vacuum is regulated so that temperature is always kept below about 185 F. The temperature is kept low enough that all steam added by or through the venturi tube is removed from the digester without removing the water of the fluid mass. In this manner, there is no increasing of the concentration of the delignifying agent nor is there any lowering of the level of the fluid mass in digester. Satisfactory operations usually employ a 16" vacuum at about 135 F. to about 165 F. temperature. The time of digestion in the present process is about onequarter to about one-sixteenth of the cooking period of old standard practices depending upon the condition of pulp desired.

In a normal sulfite or acid cycle heretofore in common practice, the cook usually required about eight hours whereas in the present process, only about one-half hour is normally required. In a normal alkaline or kraft cycle, the cook usually needs about 3 hours, whereas in the novel process, only about 6 minutes to about minutes are required, depending upon the sizes of the chips and their wetness. Of course, allowance for the water in the chips should be made when adding the amount of liquor.

The present process is preferably carried into practice by using the novel digester illustrated in the drawings. As will be seen in FIG. 1, wood chips or other cellulosic material are charged into the digester. A solvent or delignifying agent is pumped into the digester to make a fluid mass with the wood chips. Such a solvent or delignifying agent may be any suitable one including those well known to the art such as aqueous solution containing sodium sulfide and caustic soda or sodium acid sulfite and caustic soda or calcium sulfite and limei After a relatively short period of time, the circulation of the fluid mass in the digester via the venturis effects a delignification of the wood chips. In practice, it has been found that a vacuum of about 12" to about 18" in the top of the digester gives satisfactory results. Steam at about to about psi injected into. the venturis This system lends itself to automation by caused the steam of fluid mass to pass through the cir' culation tubes in a satisfactory and elfective manner.

When the wood chips have been substantially or completely delignified, the fluid mass is discharged from the digester in the usual Way to a discharged pit. To assist the discharge, it is preferred to mount a venturi in the discharge outlet. The fluid mass is treated in the discharged pit in the conventional manner whereby the black liquor is separated from the pulp. It is desirable to conduct pulp containing delignifying agent to and through beaters or refiners before washing. By this technique, a greater percentage of souble lignin is removed. The pulp is then washed to remove solubles including soluble lignin. As is well known, the black liquor may be treated in a chemical recovery system to recover chemical and byproducts.

The beaters or refiners break up the mass of fibers into fibers of desired length. Such beaten fibers can go directly to a paper making machine, if desired, as well known by those skilled in the art. On the other hand,

' the beaten fibers can be subjected to bleaching in a bleacher in accordance with conventional practice. The bleached pulp can be converted into paper on any standard paper making machine. The general schematic outline of treatment may be seen from the flow sheet illustrated in FIG. 1.

Preferred embodiments of the novel digester are illustrated in FIGS. 2 to 5 of the drawings. The novel digester has a shell somewhat similar to those employed in conventional digesters. While it is preferred to use an unlined shell of suitable material to resist alkali or acid as shown in the drawings, a lined shell can be employed as in ordinary practice. The shell as well as the circulation tubes or pipes and the venturis are preferably made of corrosion-resisting metals or alloys. For example, the shell and auxiliaries can be made of the alloys disclosedin US. Patents Nos. 1,316,817; 1,339,378;

1,587,614; 1,841,752; 2,200,229; 2,553,330; 2,678,881 and 2,777,766. As those skilled in the art know, various prior publications and patents disclose the kind of metals and alloys to be used in various parts of equipment in a pulp and paper mill. In the issues of Paper Trade Journal, November 9, 16 and 23, 1950, for instance, H. O. Teeple has disclosed various alloys in his article entitled The Use of Nickel Containinig Alloys in the Pulp and Paper Industry and has referred to other publications in this field.

The novel digester has a cylindrical portion C on top of which an appropriate head H is welded or otherwise suitably secured and on the lower end of which an appropriate bottom cone B is likewise welded or suitably fastened. It is preferred to mount the digester on a plurality of legs G which can be united with the bottom cone B in accordance with conventional practice, although any appropriate mode of mounting may be used. In the head H, a charging port P is provided which can be closed with a removable cover R. In the embodiment illustrated in FIG. 2, a plurality of bolts and nuts N are used to hold the cover in a closed condition whereas in the embodiment illustrated in FIG. 4, a screw type of cover may be utilized. Continuous charging may be used by employing a suitable star type of valve. A vapor outlet V is likewise incorporated in the head which is connected in the ordinary manner to a standard suction pump and condenser (not shown) whereby steam, volatile constituents evolved during the digesting, vapors and air or gas may be withdrawn during the delignification of the pulp. In this manner, a desired degree of vacuum can be drawn and maintained, such as about 14" to 16" of mercury. A well known top relief valve and valve T may be mounted in the head. Normally such auxiliaries are not required as pressures do not increase in the novel process. Other well-known auxiliaries including a sampling cock, a thermometer Well, etc., are incorporated in the digester. A rotating fan may be provided at a point just below the vapor outlet to take care of any foaming or priming of heavily saturated liquor. However, since all of these auxiliaries are so well known to those skilled in the art, it is not necessary to illustrate them. i

To remove the contents of the digester, a valved discharge line W is connected with the lowermost portion of the bottom cone. To assist in facilitating the discharge of the digester, it is preferred to incorporate a supple: mentary discharge venturi Y in the discharge line going to a regular blow pit (not shown). The venturi may be constructed in any suitable manner as Well known by those skilled in the art, although it is preferable to have the proportions so that L (the length of the tube extending from the entrance to the venturi to the throat thereof) equals the discharge tube or pipe diameter D, l (the length of tube extending from the throat to the exit end of the venturi) equals about one-half to about one-third of the tube diameter D and d (the diameter of the throat) equals about 0.4 the tube diameter D. An appropriate steam jet I is located in line with the venturi throat having a diameter d.

To effect a rapid circulation of the fluid mass within the digester, a plurality of circulation pipes K are mounted adjacent to the inner wall of the cylindrical shell C. These pipes terminate in open ends near the inner surface of the bottom cone B as may be clearly seen from FIGS. 2 and 4. Near or at the top of each tube, a venturi is provided. The embodiment shown in FIG. 2 depicts a horizontal leg Within which the venturi is incorporated, Whereas in FIG. 4, the venturi is mounted near the top of the tube. In FIG. 2, a steam jet SI enters the horizontal arm of each circulating tube K and terminates in an open end near the entrance or inlet of the venturi. The tube extending from an inlet 1 has the form of a truncated cone. At the smallest end of this cone is a throat TH of the venturi.. From the throat, the pipe again takes the form of a truncated cone with an outlet 0 being equal in diameter to that of the tube. While the venturi may be made up of sections or parts coupled or connected together, it is preferred to weld or suitably fasten the parts together as depicted in FIG. 2. The venturi outlet is connected with or joined to a return bend RB with an open leg extending downwardly into the interior or central portion of the digester. The same venturi effect is obtained in the embodimentillustrated in FIG. 4 by incorporating the venturi within the tube and near the top thereof leading to the horizontal return bend RB. A steam jet SI enters each circulating tube and terminates adjacent to an entrance or inlet I of the venturi. From the inlet a truncated cone portion extends to a throat TH and from the throat another truncated cone extends to an outlet 0 which joins the circulating tube. In FIG. 2, there are six circulating tubes (as may be clearly seen in FIG. 3) whereas in FIG. 4, there are only four as clearly shown in F G. 5. The stream jets SI are supplied with steam under pressure from a circular steam supply CS which is controlled by a main valve MV. For controlling each individual steam jet, a small control valve SV is provided as clearly illustrated in FIGS. 3 and 5. While any appropriate venturi proportions and dimensions may be used, it is preferred to use the following for the venturi in the circulating tubes for 12 to 16 foot diameter digesters: i

D equals about 8 to about 24 inches L equals about 8 to about 24 inches 1 equals about 2.3 to about 8 inches d equals about 3.2 to about 9.6 inches In the operation of the novel digester, a charge of wood chips or other suitable fibrous material is introduced via the charging port. After charging with wood chips, a fluid or aqueous solvent or delignifying agent is poured or flowed into the digester to cover the chips and to fill up the digester to an appropriate level such as indicated in the drawings. The delignifying agent may ,pit preferably against a target.

agent, any steam or water vapor, etc.

be any suitable one as selected by those skilled in the art or any standard one regularly used in conventional operations of pulp making. Such delignifying agents are full disclosed in prior publications and patents. For instance, the textbooks entitled The Manufacture of Pulp & Paper published by McGraw-Hill (1922, 1927) and Pulp and Paper Chemistry and Chemical Technology (Second Edition) written by James P. Casey and published by Interscience Publishers, Inc. (1960), describe fully fibrous materials suitable for pulping, delignifying agents suitable for delignification of wood chips or other fibrous materials, digesters with appropriate equipment and auxiliaries and operations and procedures using the foregoing materials, agents, digesters, auxiliaries, etc. Such textbooks contain other references to the prior art which may be used.

For satisfactory operation, a vacuum is drawn and maintained at about 14" to about 18 of mercury. By so doing, air, gas and the like are withdrawn from top of the fluid mass in the digester. Simultaneously or subsequently, steam can be introduced into the steam jets in the venturis. The flow of steam will cause a flow of fluid through the circulating tubes and back into the central portion of the digester. By continuing the flow of steam, the fluid mass can be heated to a selected temperature. For instance, it has been found that a temperature of about 165 F. gives satisfactory results and that about 20 to about 35 pounds per square inch (gauge) [i.e., 35-50 p.s.i.] can produce this temperature.

By continuing the circulation in the digester via the venturis, the delignification occurs in a relatively short period of time. Usually, it is complete or substantially so in a fraction of the time ordinarily required in the conventional pulping procedures. The venturis are particularly effective in delignifying wood chips or other fibrous material. It is believed that the venturi action is as follows:

The Wood chips and hydrolyzing liquor form a fluid mass that has movement imparted to it by means of the venturi action due to the differential of pressure between the pressure within the digester and the pressure of the steam jet. This pressure causes a higher penetration of the solvent into the cellular structure of the wood chips to effect a removal of the lignin while the fluid mass passes through the venturi tube. The removed and/ or dissolved lignin is washed or scrubbed off on impinging the top of the surface of the fluid mass in the digester causing fresh surfaces to be rapidly exposed for fresh penetration by the continuous circulation of the fluid mass through the venturi tube and then through the body of :the digester for a return to the inlet of the venturi tube. Regardless of what is the mechanics of the action of delignification, it has been found as a practical matter that the venturis function faster and better than conventional procedures.

When the wood chips are completely delignified or substantially so, the fluid mass in the digester can be discharged. The vacuum is discontinued and the steam is turned off to the venturis. The discharge valve is opened at the bottom and the fluid mass is discharged into a blow To assist and facilitate the discharge, steam is introduced into the supplementary discharge venturi. The black liquor, i.e., the solution containing the lignin, hydrolyzed hemo-celluloses and soluble components is drained from the pulp. Such black liquor is preferably sent to a chemical recovery system in order to recover any desirable materials. The remaining pulp is beaten and refined, washed thoroughly,

screened, bleached, if desired, and finally made into paper.

The use of vacuum makes it possible to recover any volatile constituents evolved during the digestion operation, any sulfur dioxide liberated from the delignifying It is particularly to be noted that materials which have been recovered inelude pine oil and terpenes.

A series of tests were made carrying the present invention into practice and using a small vessel functioning as a digester and connected by a suitable hood to a condenser, receiver and aspirator which maintains about 14" to about 16" of vacuum and which keeps the fluid mass in the vessel at or below about 165 F. A charge of delignifying liquid was placed in the vessel with a charge of screened saw dust. The vessel was provided with a venturi of about bore constricted to about A". Steam was then passed through the venturi at from about 12 to about 20 p.s.i. Delignification started immediately after a flow commenced through the venturi tube as evidenced by the fluid turning black at the discharge end of the venturi. Substantially complete delignification resulted in a short period of about 6 to about minutes of treatment.

Test runs were made using the same digesting liquor in a large wooden vessel provided with hood, a pressure or vacuum gauge, thermometer, and a 4"-1 /2 venturi and a steam line. Steam going to the venturi is at about to about 100 p.s.i. The vessel is connected to a condenser and aspirator which maintains a vacuum of about 14"l6". Wood chips of about /2" x 1" (approx) are charged into the vessel and are covered with a digesting liquor making a fluid mass. The amount of chips is about one part by dry weight and the liquor is about six parts by weight. The concentration of chemicals in the liquor is such that the liquor has a specific gravity of 1.20 B. The temperature of the fluid mass is maintained below about 165 F. Substantially complete delignification was made in a short period of less than about 30 minutes. There was substantially no degrading the cellulosic fibers. A yield of cellulose above of the wood chips was attained. The wood chips were pine chips as normally used in paper making practice.

The cooking liquors used for the various test runs had the following compositions:

(l) About 41 gallons of liquor contained about 1 gallon H O, about 260 g. NaOH and about 96 g. Na SSI-I O.

(2) About 1 gallon of liquor contained about 1 gallon H O, about g. 85% NaOH, about 48 g. Na S.9H O.

(3) About 1 gallon of liquor contained about 1 gallon H O, about 20 g. 85% NaOH, and about g. NaHSO3.

(4) About 1 gallon liquor contained about 1 gallon H O, about 20 g. 85% NaOH, and about 80 g. NaHSO Each cooking liquor was used in a different test run as those skilled in the art will understand. 7

The weight of air dried saw dust or chips was about four times the weight of the greatest chemical concentration in all tests and was fluid enough to pass through the venturi as delignification proceeded so rapidly that saw dust or chips were still floating on the initial admission of steam to the venturi and flow. They all were reacted upon and in the fluid mass flow within about five minutes as observed through the sight glass in the hood.

For the purpose of giving those skilled in the art a better understanding of the invention and/ or a better appreciation of the advantages of the invention, the following illustrative examples are given:

Example I A pilot digester having a diameter of almost four feet and a depth of about 6 feet was provided with a suitable modified venturi tube as more fully illustrated in the drawings and described herein. The pilot digester was charged with standard-sized pine chips having a variable length of about 1 to about 3 inches, a width of about /2 to about 2 inches and a thickness of about to about 4 inch. The weight of the charge was about 88 pounds on a bone dry basis.

The solvent for the lignin and other solublecomponents comprised a liquor containing caustic soda and sodium sulfide. It was essentially a 20 B. standard pounds of pulp on bone dry basis.

kraft liquor. The charge of cookingliquor amounted to about 472 pounds and the ratio of liquor to the wood chips was about 5.5: 1.

under pressure to the venturi tube, a flow of liquor and chips (herein termed a fluid mass) through the venturi was eflfected. This fiow was from the bottom of the digester to the top and then discharged above the fluid mass to impinge upon the uppermost surface and finally down again to the bottom causing a complete circulation of the fluid mass. The cycle was repeated over and over again in a continuous manner. The flowing liquor carried the wood chips which were subjected to the action of the forces including those of pressure and vacuum in the venturi. Throughout the cycle of flow, the chips were also subjected to the action of the cooking liquor. The cooking action and the flow through the venturi was continued for a relatively short period of time when the chips were found to be completely cooked or digested in the form of pulp. In this illustration, the period of time was Example 11 About 35 pounds of screened standard-sized Canadian spruce chips were charged to the pilot digester used in Example I. About 375 pounds of cooking liquor containing about 25 pounds of commercial NaHSO and about 8 pounds of 65% NaOH were introduced into the digester. The ratio of liquor to chips was about 15 to 1.

The chips were processed for about five minutes in the same manner set forth in Example I. The steam had a pressure of about 100 p.s.i. and the temperature of the cooking liquor was about 135 F. The pulp was washed in five waters for about one minute in each wash water to remove chemicals, etc. The washing was continued until the wash water was neutral to litmus. After washing, the pulp was refined for an additional seven minutes in a Knapp-Monarch liquidizer. The yield was 21.5 In other words, the yield was about 61% of the wood chips charged to the digester. The lignin content was about 2.5%

Example 111 About 77 pounds of planar mill shavings of Virginia pine wood were charged to the digester used in Example I. These shavings varied in size but, generally speaking, had a width of about 1 to about 4", a thickness of about to about A and a length of about 2" to 16". Ex-

. tra cooking liquor was required because of the loose packing of the shavings. About 864 pounds of cooking liquor were used having substantially the same composition as in Example 11. The shavings were processed for about five minutes using steam at a pressure of about 100 p.s.i. and the cooking liquor had a temperature of about 165 F. The pulp was washed until neutral with wash water as is usually done. The washed neutral pulp was refined for a total period of time of about 11 minutes in a Knapp- Monarch (H & M) liquidizer The yield was about 68% of the wood shavings charged. The lignin content was about 18%.

Example I V A digester is used having a diameter of about 42 inches inside and a depth of about 72 inches and provided with per liter.

a truncated conical bottom. The venturi tube in the digester is about eight inches in diameter and is connected by suitable pipes, valves, etc., to a boiler capable of delivering steam at about 100 p.s.i. An 8-inch vapor outlet is located in the top of the digester and is provided with a condenser and pump as well as necessary gauges, valves and fittings.

About 125 pounds of pine wood chips as used in standard mill practice was charged into the digester together with about 510 pounds of standard white cooking liquor containing about 200 grams of NaOH and about 5 grams of NA S per liter. The pump was started and vacuum built up to about 14 inches when steam was admitted to the venturi at about 100 p.s.i. This pressure dropped down to about p.s.i. when the steam line to the venturi jet was fully open. The vacuum fluctuated from about 14 inches down to a lower value and back to about 14 inches over a short period of about 27 minutes. The variation in the vacuum was due to fluctua tions on the electrical power line. Circulation of the fluid mass was continuous from the opening of the steam jet valve to the shut down at the end of 27 minute run. The contents of the digester was blown down to a suitable blow tank. The net weight of processed wet chips recovered was about 160.2 pounds and the maximum fluid mass temperature was about 188 F.

The original wood chips contained about 33.3% of moisture and, therefore, about 83.25 pounds of dried wood chips were charged. The net weight of dried process chips was about 52.2 pounds. The ratio of liquor to dry wood was about 6 to 1 on a weight basis. The theoretical yield of pulp having about 62% cellulose content would be about 51.62 pounds. The actual yield in this test was 52.2 pounds.

The average best yield in common practice is considered to be 47% of wood used or about 36.93 pounds based on the weight of wood in this run. The actual yield of 52.2 pounds of pulp against 37 pounds based on standard practice shows that .the novel process produces a 41% increased yield over standard practice.

The fibers produced by the novel process when examined under a 750 power microscope show the fibers are full length, non-degraded and lignin free.

Example V Using a test vessel holding about one gallon with a inch venturi and other auxiliaries described heretofore, about 112.5 grams of ground pine wood was charged. The fibers or particles had a diameter of about 1 to 2 mm. and a length up to about 1 to 6 mm. About 750 grams of cooking liquor were added. Steam at about 10 p.s.i. was admitted to the venturi for about 1 /2 minutes. The digester was under a vacuum of about 12 inches. The cooking liquor contained about 200 grams of Na SSI-I O and about grams of NaOH (65 per liter. The test yielded about 80.2 grams of dried pulp with 38% of the lignin of the wood removed.

Example VI inch vacuum. The delignifying media in this case contained 200 grams of NaOH and 200 grams of N21 S.9H O Steam was admitted for about five minutes during which time the fluid mass of chips and liquor circulated freely and rapidly through and around the digester. At the end of the run,-the fluid mass was removed from the digester and was washed to neutral to litmus. The resulting pulp was beaten in a Knapp- Monarch liquidizer for about three minutes. About 96% of the original lignin in the wood was removed. The

.pulp formed a nice mat and cleared the screen entirely. A yield of about 67.3% oven-dried pulp was produced.

About two milliliters of pine oil or terpenes was recovered from the condensate.

It is to be observed that the present invention provides an improved process for pulping wood chips or fibrous materials under a partial vacuum while at the same time subjecting the fluid mass to pressure in the venturi and then to reduced pressure at the throat of the venturi which effects an almost instantaneous delignification of the cellulosic material, or differential pressures suflicient to cause the circulation of the fluid mass through the venturi within the digester.

Furthermore, the invention provides an improved process of pulping involving a more rapid cook of cellulosic material at a temperature below about 185 F. and preferably between about 135 to 165 F. with little or no degrading of the cellulose fibers thereby effecting a higher yield of cellulose from the charged wood than is possible under the customary digesting methods. Vari- .ous steam pressures may be used in the jet of the venturi and may preferably range from about p.s.i. to about 100 p.s.i. and for best results are about p.s.i. to about p.s.i. The vacuum is maintained within the digester .from about 12 inches to about 18 inches of mercury and preferably from about 14 to about 16 inches. In special applications where some degradation of the pulp is desired, the digester is operated at from about 40 or 60 to about p.s.i. and the steam pressure used in the jet .of the venturi is about 250 p.s.i. to 600 p.s.i. and is preferably about 450 p.s.i. to about 600 p.s.i. so as to get the maximum benefits due to differential pressures.

Moreover, the invention provides an improved process of pulping involving the use of a vacuum whereby aromatic oils usch as oil of pine or cedar, terpenes, etc., are recovered from the condensate.

It is also to be noted that the present invention provides an improved vacuum-venturi pulping process in which conventional blowing of the cook is not required with the attendant mal-odors of a paper plant and which removes a civic objection to prior methods; in which inexpensive tanks may be used for the process tank or digester instead of very costly digesters which are also expensive to maintain; in which greater control of the cellulose produced may be had by shortening or lengthening the cook period by minutes instead of hours; in which the amount of chemicals may be greatly reduced for each batch giving a great saving or at least twice as much wood and up to six times thereof may be pul ed using the same amounts of chemicals now customarily U employed; in which an increased yield of cellulosic pulp from standard practice of about 42 to about 48% to about 58% to about 64%; and in which manufacture of either kraft or alkaline papers may be made in the same equipment.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. It is conceivable that an amount of fiber degradation may be desired for certain types of paper that may only be obtained at considerably higher temperature cook than this method usually employs. This effect can be secured using existing digesters equipped with one or more of the modified venturis to provide positive circulation and permitting the pressure within the digester to build up to 60 to 100 p.s.i., using superheated steam at 400 to 600 p.s.i. in the steam jet. Excess vapors are carried off via the condenser and pump. This cycle uses the same principles of applied higher pressure in the venturi discharging to a much lower pressure within the digester. The beneficial action of the venturi tube is utllized in this case the same as in the vacuum pressure cycle. The charge of wood chips and liquor would or may be atmospheric temperatures as before. The-time of cock will be approximately the same, to wit, about 5 to about 30 minutes. The fluid mass will be uniformly heated without scorching and the desired temperature and pressure will be rapidly built up by providing a valve in the vapor line to the condenser. Pressure is maintained by regulating this valve to the condenser. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

I claim:

1. In a combination venturi-vacuum process of digesting pulp involving a more rapid pulping of chips of wood, cellulosic materials and fibrous compositions resulting in a maximum yield of cellulose under combined venturivacuurn influence of lower temperatures from the treatment of larger quantities of raw materials in a shorter period of time than heretofore with the same amount of chemicals used as delignifying medium and without degradation of fibers, the improvement which comprises charging a digester with a fluid mass containing a delignifying medium and chips of the group consisting of WOOd, cellulosic materials and fibrous compositions and leaving an air and vapor space within the top of the digester and above the surface of the fluid mass; adjusting the fluid mass to provide an excess of fluid in proportion of several times the weight of the chips; sealing the digester against the atmosphere; applying a vacuum to said air and vapor space to maintain a pressure less than atmospheric pressure; establishing a plurality of small, separated and independent streams of fluid mass in the peripheral portion of said digester; moving each of said streams at high velocity upwardly; providing a venturi located in the upper part of each of said streams and having a single jet centrally located in the venturi throat to apply a solid jet of steam under pressure; disintegrating said chips in the fluid mass in each of said streams by continuously passing steam under pressure of about 10 p.s.i. to about 600 psi. through each single jet in each venturi; forcing fluid mass from the plurality of streams under pressure from each venturi and impinging the same downwardly upon the top surface of the fluid mass in the digester to further disintegrate said chips while maintaining a vacuum to place each venturi under differential pressure and to remove volatile components released during del-ignificati-on, water vapor, and excess steam to prevent dilution of the delignifying medium; maintaining a moderate temperature in said fluid mass of about F. to about F.; continuing disintegration of chips in each of said streams by continuously passing steam under pressure through each centrally located jet 1n each venturi and by continuously impinging chips upon the top surface of the fluid mass in the digester; discontmuing the passage of steam under pressure through each et after the expiration of a short period of time within a range of about 3 minutes to about 30 minutes when said chips in the fluid mass are converted into pulp and the desired degree of delignification has been attained; and discharging said fluid mass from said digester whereby pulp is produced.

:2. In a combination venturi-vacuum process of digesting pulp involving a more rapid pulping of chips of wood, cellulosic materials and fibrous compositions resultlng in a maximum yield of cellulose under combined ventun-vacuum influence and lower temperatures from the treatment of larger quantities of raw materials in a 13 sealing the interior of the digester from the atmosphere; applying a vacuum of at least several inches of mercury to the air and vapor space in the top of the digester to remove volatile components released during delignification, Water vapor and excess steam evolved from said fluid mass to prevent dilution of the de'lignifying liquor; establishing a plurality of small, separated and independent streams of fluid mass upwardly in the peripheral portion of the digester and downwardly in the central portion; passing the upwardly flowing streams through a plurality of independent and separated paths; locating a venturi in the upper region of each path provided with a single jet centrally located with respect to the throat of the venturi for producing maximum effect by a solid jet of steam under pressure; admitting steam under pressure via the single, centrally-located jet in each venturi to subject chips in the fluid mass to pressure and a disintegrating action while passing throu-gh the venturi throat under the influence of said delignifying liquor and while under a diffenential pressure on the fluid mass effecting deeper penetration of the delignifying liquor in the chips; controlling the steam pressure within a range of about 10 p.s.i. to about 600 p.s.i. in each venturi; maintaining a moderate temperature in said fluid mass up to about 135 F. to about 165 F.; impinging the heated fluid mass in each stream issuing under steam pressure from each venturi onto the top surface of said fluid mass in the digester while maintaining a vacuum in the air and vapor space to place each venturi under ditferential pressure; continuing the rapid disintegration of chips under differential pressure in the fluid mass flowing upwardly in each stream through a venturi in the peripheral portion of the digester and impinging onto the top surface of the fluid mass in the central portion of the digester; and discontinuing said rapid disintegration within a relatively short period of time Within a range of about 3 minutes to about 30 minutes via each of said venturi While subjecting port-ions thereof to the action of the venturi in each stream and the pressure of the steam until substantially complete delignification of the chips is effected and a high grade pulp is produced.

References Cited by the Examiner UNITED STATES PATENTS 1,784,197 '12/ 30 'N-at'wick 162-243 1,859,845 5/32) Rue.

1,905,731 4/33 McKee 162-243 1,954,625 4/ 34 Hellstrom 16-2--24 3 1,971,241 8 34 Weitzel 16259 1,982,002 11/34 Hatch 162-59 2,344,047 3 44 Lowe 162-243 DONALL H, SYLVESTER, Primary Examiner.

JOSEPH B. SPENCER, MORRIS O. WOLK, Examiners. 

1. IN A COMBINATION VENTURI-VACUUM PROCESS OF DIGESTING PULP IMVOLVING A MORE RAPID PULPING OF CHIPS OF WOOD, CELLULOSIC MATERIALS AND FIBROUS COMPOSITIONS RESULTING IN A MAXIMUM YIELD OF CELLULOSE UNDER COMBINED VENTURIVACUUM INFLUENCE OF LOWER TEMPERATURES FROM THE TREATMENT OF LARGER QUANTITIES OF RAW MATERIALS IN A SHORTER PERIOD OF TIME THAN HERETOFORE WITH THE SAME AMOUNT OF CHEMICALS USED AS DELIGNIFYING MEDIUM AND WITHOUT DEGRADATION OF FIBERS, THE IMPROVEMENT WHICH COMPRISES CHARGING A DIGESTOR WITH A FLUID MASS CONTAINING A DELIGNIFYING MEDIUM AND CHIPS OF THE GROUP CONSISTING OF WOOD, CELLULOSIC MATERIALS AND FIBROUS COMPOSITIONS AND LEAVING AN AIR AND VAPOR SPACE WITHIN THE TOP OF THE DIGESTER AND ABOVE THE SURFACE OF THE FLUID MASS; ADJUSTING THE FLUID MASS TO PROVIDE AN EXCESS OF FLUID IN PROPORTION OF SEVERAL TIMES THE WEIGHT OF THE CHIPS; SEALING THE DIGESTER AGAINST THE ATMOSPHERE; APPLYING A VACUUM TO SAID AIR AND VAPOR SPACE TO MAINTAIN A PRESSURE LESS THAN ATMOSPHERIC PRESSURE; ESTABLISHING A PLURALITY OF SMALL, SEPARATED AND INDEPENDENT STREAMS OF FLUID MASS IN THE PERIPHERAL PORTION OF SAID DIGESTER; MOVING EACH OF SAID STREAMS AT HIGH VELOCITY UPWARDLY; PROVIDING A VENTURI LOCATED IN THE UPPER PART OF EACH OF SAID STREAMS AND HAVING A SINGLE JET CENTRALLY LOCATED IN THE VENTURI THROAT TO APPLY A SOLID JET OF STREAM UNDER PRESSURE; DISINTEGRATING SAID CHIPS IN THE FLUID MASS IN EACH OF SAID STREAMS BY CONTINUOUSLY PASSING STEAM UNDER PRESSURE OF ABOUT 10 P.S.I. TO ABOUT 600 P.S.I. THROUGH EACH SINGLE JET IN EACH VENTURI; FORCING FLUID MASS FROM THE PLURALITY OF STREAMS UNDER PRESSURE FROM EACH VENTURI AND IMPINGING THE SAME DOWNWARDLY UPON THE TOP SURFACE OF THE FLUID MASS IN THE DIGESTER TO FURTHER DISINTEGRATE SAID CHIPS WHILE MAINTAINING A VACUUM TO PLACE EACH VENTURI UNDER DIFFERENTIAL PRESSURE AND TO REMOVE VOLATILE COMPONENTS RELEASED DURING DELIGNIFICATION, WATER VAPOR, AND EXCESS STEAM TO PREVENT DILUTION OF THE DELIGNIFYING MEDIUM; MAINTAINING A MODERATE TEMPERATURE IN SAID FLUID MASS OF ABOUT 135*F. TO ABOUT 185*F.; CONTINUING DISINTEGRATION OF CHIPS IN EACH OF SAID STREAMS BY CONTINUOUSLY PASSING STEAM UNDER PRESSURE THROUGH EACH CENTRALLY LOCATED JET IN EACH VENTURI AND BY CONTINUOUSLY IMPINGING CHIPS UPON THE TOP SURFACE OF THE FLUID MASS IN THE DIGESTER; DISCONTINUING THE PASSAGE OF STEAM UNDER PRESSURE THROUGH EACH JET AFTER THE EXPIRATION OF A SHORT PERIOD OF TIME WITHIN A RANGE OF ABOUT 3 MINUTES TO ABOUT 30 MINUTES WHEN SAID CHIPS IN THE FLUID MASS ARE CONVERTED INTO PULP AND THE DESIRED DEGREE OF DELIGNIFICATION HAS BEEN ATTAINED; AND DISCHARGING SAID FLUID MASS FROM SAID DIGESTER WHEREBY PULP IS PRODUCED. 